Filter beds formed from one or more layers of filter media have been employed in a variety of known filters for filtering water or wastewater to remove impurities from liquids. For example, filter beds of granular media have been used in upflow filters, downflow filters as well as other type of filters including bi-flow filters. After the filter has been operating for a while, it is necessary to wash the filter bed to remove the impurities trapped in the filter beds during the filtration mode. Various methods have been used to wash the filter bed including but not limited to the steps of: (i) liquid only wash; (ii) air only wash; (iii) liquid and air concurrently; (iv) liquid only followed by air only; (v) air only followed by liquid only; and, (vi) liquid and air concurrently followed by liquid only.
It is important that the washing fluid is thoroughly distributed through the filter bed during the washing mode in order to remove the impurities trapped in the filter bed during operation of the filter in the filtration mode. Various underdrains and flumes have been used in an attempt to distribute the washing fluids uniformly throughout the filter beds. Significant problems can be encountered by prior art devices. For example, a washing procedure that employs simultaneous liquid and air washing fluids is highly susceptible to mal-distribution of the washing fluids through the filter bed. Typically, in systems using this type of washing procedure, the filter bed is disposed above the underdrain. The underdrain often consists of a plurality of underdrain laterals placed in a side-by-side fashion. The underdrain laterals direct gas and liquid through the filter bed during the washing mode. The underdrain laterals are in fluid communication with a flume. The flume receives the washing fluids (i.e., washing gas and washing liquid) from their source and directs these fluids to the underdrain laterals. The washing gas/washing liquid interface in these flumes is often low, i.e., close to the bottom of the flume this is due to the properties of the washing gas (e.g., air) and their effect on the washing liquid including forcing the uppermost level of the washing liquid (e.g., filtered or unfiltered water) downwardly. This is undesirable as it limits the area available for the washing liquid resulting in relatively high liquid washing flow velocities down the length of the flume which in turn causes mal-distribution of the washing fluids to the underdrain and ultimately the filter bed.
One proposed solution to the low gas/liquid interface problem in the flume is to provide a flume with a bottom that is lower than the bottom of the filter bed. An example of this type of construction is shown in FIG. 3 of U.S. Pat. No. 6,312,611. Alternatively, separate members have been provided for conveying and distributing liquid and gas separately in an attempt to overcome the low gas/liquid interface problem. Examples of these types of devices are illustrated in FIGS. 4 through 7 of U.S. Pat. No. 6,312,611.
Another proposed solution to the low gas/liquid interface problem has been to provide at least one flume liquid metering orifice (i.e., a closed perimeter opening) in a particularly shaped baffle or stand-pipe. Examples of these structures are shown in FIGS. 10 through 20 of U.S. Pat. No. 6,312,611.
These designs still have significant problems. The washing liquid/washing gas interface still can lead to significant mal-distribution of the washing fluids in the filter bed. Further, where a flume has both a washing liquid and a washing gas present the area for each is limited by the area of the other.